Kouetsu Kirijo is the father of Takeharu Kirijo and the grandfather of Mitsuru Kirijo. He was once the head of the Kirijo Group before his death. 14 years ago Kouetsu Kirijo began the experimentation of Shadows in order to learn and harness their powers to create a time manipulation device. He assembled a team of top scientists and for several years he was able to obtain a significant amount of Shadows. Over the years he became a Nihilist and began preparing for The Fall. 10 years ago during the final stages of the experiment they lost control of the Shadow's power. He and the rest of the scientists lost their lives during the accident.

According to the Persona 3 Club Book Kouetsu Kirijo originally came across a Plume of Dusk, and started his research into it for benevolent purposes. As more was learned from it about Nyx and more Plume of Dusks were collected, however, eventually drove Kouetsu mad.